This past Friday, shuttle launch director Mike Leinbach scrubbed the scheduled 3:37pm eastern launch of Endeavour as engineers investigated a failure to one of the heating circuits serving an Auxiliary Power Unit (APU). The heaters prevent the APU's hydrazine from freezing while the shuttle is in orbit.

Engineers believed the problem was associated with a switchbox at the spacecraft's aft end or an electrical short in the wires that connect to the switchbox. On Saturday, engineers entered the shuttle's aft comparent to examine the malfunctioning system, and on Sunday, NASA announced that as testing continued, the shuttle would not launch on Monday as was previously indicated.

According to the NASA website, the issue seems to involve the aft load control assembly-2 (ALCA-2), a box of switches controlling power feeds. "That basically means the power is not getting out to the heaters that weren't working on launch day," Space Shuttle Program Launch Integration Manager Mike Moses said.

The six Endeavour astronauts have returned to Johnson Space Center in Houston for more training before they go back to Kennedy for the next launch attempt. ®